The embodiments described herein relate generally to the field of transmissions for hybrid vehicles, and more particularly to powertrain systems including a clutchless transmission for improving powertrain performance in high-performance vehicles.
Automotive transmissions are used to transfer power from an engine to the wheels of a vehicle. In particular, known transmissions allow the selection of multiple gear ratios to modulate the power and speed that are applied to the wheels. Known manual transmission and powertrain systems include a clutch to selectively disengage the engine from the transmission to facilitate selection of different gears (i.e., “shifting” of gears). In use, known clutches equalize the speed of the engine and the shafts and/or gears within the transmission during shifting of gears. Known clutches, however, increase the complexity of the powertrain (e.g., by including additional parts) and decrease the overall efficiency of the transmission. For example, the efficiency of known transmissions is generally in the approximate range of 84-92 percent. Moreover, as much as 25 percent of the overall power losses in known transmissions can be attributed to the clutch. As one example, for known wet clutches, some power losses come from the fluid within the case, which form an internal resistance. During operation, the fluid produces a shearing force that generates a drag torque, which becomes a drag loss.
Moreover, although transmissions have been studied and used for decades, there remains a need for improved powertrain systems for gas-electric hybrid vehicles. The development of hybrid vehicles has increased as the impact of anthropogenic climate change has become a global concern. For example, European and American regulators have instituted yearly targets for fuel economy and carbon emissions. Car manufacturers that do not meet these targets face heavy fines. Of additional concern to automobile manufacturers is Corporate Average Fuel Economy (CAFE). CAFE targets in the U.S., and in similar programs around the world, incentivize fuel efficiency and penalize manufacturers that fail to meet emissions goals.
Known hybrid gas-electric vehicles are one solution to meet the demand for greater fuel efficiency and reduced emissions. Specifically, known hybrid gas-electric vehicles can increase fuel economy by leveraging the electric motor when the internal combustion engine (ICE) is not operating efficiently. For example, in known “mild hybrid” configurations, a battery and small electric motor (EM) help power the vehicle so the ICE can shut off when the vehicle stops. Known “full hybrid” configurations use larger EMs and batteries that can independently power the car for short times and often at low speeds. Known hybrid gas-electric vehicles include a variety of different transmissions and/or powertrain configurations to facilitate the use of both the ICE and the EM. For example, some known hybrid vehicles are “parallel hybrid” vehicles, which rely on a mechanical linkage between two power sources (the linkage being located either pre- or post-transmission). The linkage allows either or both power sources to accelerate the vehicle, allows an EM to regenerate upon deceleration, and allows the ICE to charge an EM while stationary. Known parallel hybrid powertrain systems, however, are mechanically complex, have increased mass, and do not facilitate operating the ICE at peak efficiency when compared with other hybrid approaches. Other known hybrid vehicles employ a “series hybrid” powertrain system. Series hybrid systems allow an ICE to operate at its most effective speed, and thus have the benefit of reduced ICE sizing, improved ICE efficiency, and a short charge path. The performance of known series hybrid vehicles, however, is limited by the ability of the batteries and charging circuitry to supply power to the EM. Yet other hybrid vehicles employ a “through the road” (or TTR) powertrain system. Known TTR systems include one driven axle that is motivated by one power source, while the other axle has an alternative power source. In such systems, the road is used as the link between front and rear wheels, thus energy can only be transmitted between axles while the vehicle is moving. Accordingly, one disadvantage of known TTR systems is that the batteries cannot be charged while the vehicle is physically stationary.
Moreover, although there have been advances regarding hybrid gas-electric vehicles, there are concerns about translating conventional hybrid technology to high-performance vehicles. For example, known hybrid systems often include a power-split device (PSD) to allow the ICE and EM to provide power to the wheels simultaneously. One example of a PST is a continuously variable transmission (CVT), which has been used in efficiency-oriented consumer vehicles. However, there are several potential concerns about using PSDs or other CVTs in high-performance applications. For example, in such known systems, it may be difficult to program the controls to maximize power versus torque. Additionally, known systems may produce a poor driver experience due to the loss of the distinct engine scream and gear shifting. Moreover, there may be an increased rate of repair for planetary gears because of the heating and wear of high performance driving and increased frictional losses.
Thus, a need exists for improved systems and methods which can increase fuel economy and improve performance in high-performance vehicles.